This invention relates to fluoropolymer coating compositions and substrates coated with these compositions which have improved abrasion resistance.
Fluoropolymer resins, and especially perfluoropolymer resins, are known for their low surface energy and non-stick properties as well as thermal and chemical resistance. It has long been desirable to achieve longer wearing non-stick polymer coatings on metal substrates. Of particular concern to achieving coated substrates with longer service life is the coated substrate""s ability to withstand abrasion as well as its scratch resistance. xe2x80x9cScratchxe2x80x9d is related to plastic deformation of the coating such as a cut from a knife or other metal tool. Abrasion refers to the amount of coating that is worn away as may occur by rubbing or sanding wherein the coating fibrillates and breaks away or shreds from the surface. In damaging a coated substrate, scratch may be followed by abrasion, in that a knife which causes plastic deformation of the coating, may also lead to the formation of fibrils which are subsequently worn away.
The problem of durability of the non-stick coating has often been viewed as one of adhesion of the coating to the metal substrate. If the coating is optimized for release so as to prevent food particles from sticking to it after cooking or to facilitate low friction sliding contact in other applications, almost by definition there will be difficulties in getting non-stick coatings to adhere well to the substrate.
Generally in the art, adhesion has been achieved by roughening the metal substrate prior to application of the non-stick coating so that mechanical bonding will assist chemical interaction of binders in a primer layer in promoting adhesion. Typical roughening includes acid-etching, sanding, grit-blasting, brushing and baking a rough layer of glass, ceramic or enamel frit onto the substrate. Other means of increasing adhesion and hence durability have included arc spraying a mechanically resistant layer of metallic materials onto a roughened metal substrate as disclosed in U.S. Pat. No. 5,411,771 (Tsai) and U.S. Pat. No. 5,462,769 (Tsai). Roughening substrate or applying a mechanically resistant metallic layer to improve adherence adds additional cost to the coating operation and in the case of chemical etching, there are additional costs of disposing etchant materials.
Prior efforts at achieving scratch-resistant coatings have included using harder auxiliary heat resistant resins along with perfluorocarbon polymers. Sometimes fillers such as mica and aluminum flake have been used in attempt to improve scratch resistance as disclosed in U.S. Pat. No. 4,180,609 (Vassilliou) and U.S. Pat. No. 4,123,401 (Berghmans et al.). Improved scratch resistance attributable to inorganic fillers and fillers of heat resistant polymers is disclosed in U.S. Pat. No. 5,106,682 (Matsushita). In U.S. Pat. No. 5,250,356 (Batzar), a multilayer system is disclosed which uses reinforced high build primer reinforced with small particle size aluminum oxide, an aluminum oxide reinforced intermediate coat and a conventional topcoat which provides release and yet shows reduced scratching. The above references all rely on grit blasting or frit coating of the aluminum substrate to achieve the proper adhesion.
All of the prior art solutions discussed above, while they may attempt to achieve longer life coatings by increasing adhesion or reducing scratch, do not address the mechanism of wear and how to deflect abrasive forces away from the coating surface.
The present invention addresses the need for a durable, non-stick coating with superior abrasion resistance. Specifically the invention provides for a substrate coated with a non-stick coating which resists abrasion force, the coating comprising a fluoropolymer resin containing ceramic particles to deflect the abrasion force away from the coating wherein the ratio of the thickness of the coating to the longest diameter of the ceramic particles is in the range of 0.8-2.0.
Further, the invention provides for a substrate coated with a non-stick coating which resists abrasion force, the coating comprising an undercoat and an overcoat, each containing fluoropolymer resin, the undercoat also containing ceramic particles extending from the undercoat, the overcoat telegraphing the particles extending from the undercoat through the thickness of the overcoat to deflect the abrasion force away from the coating. Preferably, the ratio of the combined thickness of the undercoat and the overcoat to the longest diameter of the ceramic particles is in the range of 0.8-2.0.
In another embodiment, the invention comprises a composition capable of forming an adherent coating to a smooth substrate and exhibiting abrasion resistance, comprising fluoropolymer, polymer binder and inorganic filler film hardener, wherein at least 30 wt % of the film hardener is comprised of large ceramic particles having an average particle size of at least 14 micrometers with the amount of ceramic particles being sufficient to provide at least 3 such particles per 1 cm length of transverse cross section of coating formed from said composition.
In another embodiment, the invention comprises a primer composition capable of forming an adherent coating to a smooth substrate and exhibiting abrasion resistance, comprising fluoropolymer, polymer binder and inorganic filler film hardener, the fluoropolymer to polymer binder weight ratio being 0.5 to 2.0:1 and the filler film hardener to fluoropolymer weight ratio being at least 1.4:1. Preferably the filler film hardener comprises large ceramic particles having an average particle size of at least 20 micrometers. More preferably the amount of large ceramic particles is sufficient to provide at least 3 such particles per 1 cm length of transverse cross section of coating formed from said primer composition.